The lac Operon

What is an operon?

In bacteria, the genes that encode and regulate the enzymes of a metabolic pathway are usually clustered together. This set of genes responsible for encoding and regulating a metabolic pathway in prokaryotes is called an operon.

The concept of the operon was first described by two scientists, François Jacob and Jacques Monod, in E. coli. Each operon consists of 4 components:

• Structural gene
• Regulatory gene
• Promoter region
• Operator region

• Promoter – The promoter region is a part of DNA that acts as a binding site for the enzyme RNA polymerase. The RNA polymerase, after binding to the promoter region, can initiate the transcription of the genes associated with it.
• Operator – The operator serves as a binding site for the repressor protein coded by the regulatory gene. Usually, the operator is located adjacent to or overlaps the promoter region of the structural gene.
• Regulatory gene – The regulatory gene codes for the repressor protein. The repressor protein is a gene regulatory protein which binds to the operator region of DNA with high affinity.
• Structural gene – The main genes responsible for coding the enzymes associated with a particular metabolic pathway are called the structural genes. All the structural genes are located adjacent to each other, with a common promoter and operator region. The RNA polymerase, on binding with the promoter region, transcribes all the structural genes into a single polycistronic mRNA.

[Polycistronic – when a single mRNA codes for more than one polypeptide chain.]

lac operon

The lac operon refers to the set of genes responsible for the metabolism of the disaccharide lactose. Jacob and Monod observed that when E. coli is grown in a medium containing glucose as the carbon source, the enzymes for the breakdown of glucose (related to the glycolytic pathway) are synthesised, but the bacteria lack the enzymes required for lactose breakdown. However, when these bacteria were transferred to a medium containing lactose as the carbon source, within a few minutes the bacteria started producing the enzymes required to metabolise lactose.

This experiment proved that the bacteria contain the information in their DNA required for the enzymes related to lactose metabolism, but these genes are only expressed when lactose is available to be utilised. It was found that lactose itself acts as the inducer for the lac operon, which activates the structural genes associated with it.

Mechanism

1) Lactose – absent

• The RNA polymerase binds to the promoter of the regulatory gene and transcribes an mRNA called repressor mRNA.
• The repressor mRNA is translated into the repressor protein by ribosomes.
• The repressor protein binds to the operator region located upstream of the structural genes.
• RNA polymerase cannot transcribe the structural genes due to the binding of the repressor protein to the operator.
• The structural genes are not transcribed; as a result, no enzymes are produced by the structural genes.

Thus, in the absence of lactose, the bacterium conserves its energy by not synthesising the enzymes required for lactose metabolism.

2) Lactose – present

• The RNA polymerase binds to the promoter of the regulatory gene and transcribes a repressor mRNA.
• The repressor mRNA is translated into the repressor protein.
• Lactose binds to the repressor protein, causing its inactivation.
• The inactive repressor cannot bind to the operator region.
• RNA polymerase binds to the promoter of the structural genes and transcribes all the structural genes into a single polycistronic mRNA.
• There are three structural genes of the lac operon located adjacent to each other, i.e. lac Z, lac Y and lac A.
• lac Z gene codes for the enzyme β-galactosidase, which catalyses the hydrolysis of the bond between glucose and galactose in the lactose molecule.
• lac Y gene codes for the enzyme permease, which increases the permeability of the cell membrane to lactose.
• lac A gene codes for the enzyme transacetylase. The exact function of transacetylase is not known, but according to some theories, it transfers an acetyl group to a toxic molecule, resulting in its inactivation.
• Together, these enzymes allow the bacteria to take up and break down lactose and use it as an energy source.

Thus, in the presence of lactose, the enzymes required for the breakdown of lactose are produced in the cell.

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